StableHLO CAPI requires building upstream dialects

[GleasonK]

Request description

Currently CAPI builds linalg and arith, etc -- but most users of CAPI (frameworks) only want StableHLO+CHLO+Serialization. Would be great to offer a stablehlo_dialect_capi which only has core APIs.

One possible separation direction:

• stablehlo_dialect_capi: Just StableHLO+CHLO+Serialization APIs, these are the things useful for PJRT frameworks.
• stablehlo_unified_capi: A separate target which builds the dialect CAPI along with the transformation passes / reference interpreter / other useful APIs, likely will be the target used for StableHLO python bindings

This will probably require splitting the serialization APIs into a separate file StablehloDialectApi.h and StablehloUnifiedApi.h. The following functions should go into the dialect API: https://github.com/openxla/stablehlo/blob/main/stablehlo/integrations/c/StablehloApi.h#L23-L117

The reference API and pass registration should go into the unified API target:
https://source.corp.google.com/piper///depot/google3/third_party/stablehlo/stablehlo/integrations/c/StablehloPasses.h

Looking at the CAPI deps today, I'm surprised that this requires upstream dialects, so definitely something to look into.

(Noted by @steeve at recent OpenXLA community meeting)